Method of making nonvolatile memory device containing carbon or nitrogen doped diode

ABSTRACT

A method of making a nonvolatile memory device includes forming a first electrode, forming at least one nonvolatile memory cell comprising a silicon, germanium or silicon-germanium diode, doping the diode with at least one of nitrogen or carbon, and forming a second electrode over the at least one nonvolatile memory cell.

RELATED APPLICATION

This application is related to Herner, et al., U.S. application Ser. No.______, titled “NONVOLATILE MEMORY DEVICE CONTAINING CARBON OR NITROGENDOPED DIODE” (Attorney Docket No. 035905/0160), filed on the same dayherewith, and hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to a nonvolatile memory array and a method ofmaking thereof.

Nonvolatile memory arrays maintain their data even when power to thedevice is turned off. In one-time-programmable arrays, each memory cellis formed in an initial unprogrammed state, and can be converted to aprogrammed state. This change is permanent, and such cells are noterasable. In other types of memories, the memory cells are erasable, andcan be rewritten many times.

Cells may also vary in the number of data states each cell can achieve.A data state may be stored by altering some characteristic of the cellwhich can be detected, such as current flowing through the cell under agiven applied voltage or the threshold voltage of a transistor withinthe cell. A data state is a distinct value of the cell, such as a data‘0’ or a data ‘1’.

Some solutions for achieving erasable or multi-state cells are complex.Floating gate and SONOS memory cells, for example, operate by storingcharge, where the presence, absence or amount of stored charge changes atransistor threshold voltage. These memory cells are three-terminaldevices which are relatively difficult to fabricate and operate at thevery small dimensions required for competitiveness in modern integratedcircuits.

Other memory cells operate by changing the resistivity of relativelyexotic materials, like chalcogenides. Chalcogenides are difficult towork with and can present challenges in most semiconductor productionfacilities.

SUMMARY OF THE PREFERRED EMBODIMENTS

One embodiment of the invention provides a method of making anonvolatile memory device, comprising forming a first electrode, formingat least one nonvolatile memory cell comprising a silicon, germanium orsilicon-germanium diode, doping the diode with at least one of nitrogenor carbon, and forming a second electrode over the at least onenonvolatile memory cell.

Another embodiment of the invention provides a method of operating anonvolatile memory device, comprising providing at least one memory cellwhich comprises a silicon, germanium or silicon-germanium diode dopedwith at least one of carbon or nitrogen, wherein the diode has beenswitched from a first higher resistivity, unprogrammed state to a secondlower resistivity, programmed state, and applying a reverse bias to thediode to switch the diode to a third resistivity, reset state, whereinthe third resistivity state is higher than the second resistivity state.

Each of the aspects and embodiments of the invention described hereincan be used alone or in combination with one another. The preferredaspects and embodiments will now be described with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating the need for electricalisolation between memory cells in a memory array.

FIG. 2 is a perspective view of a multi-state or rewriteable memory cellformed according to a preferred embodiment of the present invention.

FIG. 3 is a perspective view of a portion of a memory level comprisingthe memory cells of FIG. 2.

FIG. 4 is a graph showing change in read current for a memory cell ofthe present invention as voltage in reverse bias across the diodeincreases.

FIG. 5 a is a probability plot showing memory cells transformed from theV state to the P state, from the P state to the R state, and from the Rstate to the S state.

FIG. 5 b is a plot of the current flowing through the diode versus theapplied voltage for various diode states illustrated in FIG. 5 a.

FIG. 6 is a probability plot showing memory cells transformed from the Vstate to the P state, from the P state to the S state, and from the Sstate to the R state.

FIG. 7 is a probability plot showing memory cells transformed from the Vstate to the R state, from the R state to the S state, and from the Sstate to the P state.

FIG. 8 is a perspective view of a vertically oriented p-i-n diode thatmay be used in embodiments of the present invention.

FIG. 9 is a probability plot showing memory cells transformed from the Vstate to the P state, and from the P state to the M state.

FIG. 10 is a perspective view of a multi-state or rewriteable memorycell formed according to a preferred embodiment of the presentinvention.

FIG. 11 is a probability plot showing memory cells transformed from theV state to the P state, from the P state to the R state, and from the Rstate to the S state, then repeatably between the S state and the Rstate.

FIG. 12 is a circuit diagram showing a biasing scheme to bias the S cellin forward bias.

FIG. 13 is a circuit diagram showing one biasing scheme to bias the Scell in reverse bias.

FIG. 14 illustrates iterative read-verify-write cycles to move a cellinto a data state.

FIGS. 15 a-15 c are cross-sectional views illustrating stages information of a memory level formed according to an embodiment of thepresent invention.

FIG. 16 is cross-sectional view illustrating a diode and resistiveswitching element that may be used an alternative embodiment of thepresent invention.

FIGS. 17 a-17 c illustrate probability plots of reverse current ofdevices according to a comparative example and according to examples ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has been known that by applying electrical pulses, the resistance ofa resistor formed of doped polycrystalline silicon, or polysilicon, canbe trimmed, adjusting is between stable resistance states. Suchtrimmable resistors have been used as elements in integrated circuits.

It is not conventional to use a trimmable polysilicon resistor to storea data state in a nonvolatile memory cell, however. Making a memoryarray of polysilicon resistors presents difficulties. If resistors areused as memory cells in a large cross-point array, when voltage isapplied to a selected cell, there will be undesired leakage throughhalf-selected and unselected cells throughout the array. For example,turning to FIG. 1, suppose a voltage is applied between bitline B andwordline A to set, reset, or sense selected cell S. Current is intendedto flow through selected cell S. Some leakage current, however, may flowon alternate paths, for example between bitline B and wordline A throughunselected cells U1, U2, and U3. Many such alternate paths may exist.

Leakage current can be greatly reduced by forming each memory cell as atwo-terminal device including a diode. A diode has a non-linear I-Vcharacteristic, allowing very little current flow below a turn-onvoltage, and substantially higher current flow above the turn-onvoltage. In general a diode also acts as one-way valves passing currentmore easily in one direction than the other. Thus, so long as biasingschemes are selected that assure that only the selected cell issubjected to a forward current above the turn-on voltage, leakagecurrent along unintended paths (such as the U1-U2-U3 sneak path ofFIG. 1) can be greatly reduced.

Herner et al., U.S. patent application Ser. No. 10/955,549, “NonvolatileMemory Cell Without a Dielectric Antifuse Having High- and Low-ImpedanceStates,” filed Sep. 29, 2004, hereinafter the '549 application andhereby incorporated by reference, describes a monolithic threedimensional memory array in which the data state of a memory cell isstored in the resistivity state of the polycrystalline semiconductormaterial of a semiconductor junction diode. This memory cell is aone-time-programmable cell having two data states. The diode is formedin a high-resistivity state; application of a programming voltagepermanently transforms the diode to a low-resistivity state.

In embodiments of the present invention, by applying appropriateelectrical pulses, a memory element formed of doped semiconductormaterial, for example the semiconductor diode of the '549 application,can achieve three, four, or more stable resistivity states. In otherembodiments of the present invention, semiconductor material can beconverted from an initial high-resistivity state to a lower-resistivitystate; then, upon application of an appropriate electrical pulse, can bereturned to a higher-resistivity state. These embodiments can beemployed independently or combined to form a memory cell which can havetwo or more data states, and can be one-time-programmable orrewriteable.

As noted, including a diode between conductors in the memory cell allowsits formation in a highly dense cross-point memory array. In preferredembodiments of the present invention, then, a polycrystalline,amorphous, or microcrystalline semiconductor memory element either isformed in series with a diode or, more preferably, is formed as thediode itself.

In this discussion, transition from a higher- to a lower-resistivitystate will be called a set transition, affected by a set current, a setvoltage, or a set pulse; while the reverse transition, from a lower- toa higher-resistivity state, will be called a reset transition, affectedby a reset current, a reset voltage, or a reset pulse.

However, during the reset operation, the reverse leakage of the diode isirrevocably increased, as shown in FIG. 5 b and as will be discussed inmore detail below. The higher reverse leakage is a permanent feature ofthe diode, and the reset operation can be said to “damage” the diode.This damage is undesirable because it increases the current required to“read” large arrays of vertical diode cells, and can decrease the numberof cells programmed simultaneously, reducing the “bandwidth” into thememory chip.

The present inventors discovered that by doping the Group IVsemiconductor diode, such as a silicon, germanium or silicon-germaniumdiode with at least one of carbon or nitrogen (or both) duringfabrication, the increase in reverse or leakage current resulting fromthe reset operation is reduced by as much as four times, without anobserved degradation in any other parameters of the cell. The diodereverse or leakage current determines the power consumed during read andprogram operations, and reduction of this leakage current can reducepower, increase the bandwidth and/or improve temperature characteristicsof the cell during read and program operations. In one aspect of theinvention, the diode has a leakage current of less than 4×10⁻¹⁰ A at−5.5V in the high resistivity, reset state.

Thus, the fabricated diode is doped with at least one of carbon ornitrogen in a concentration greater than an unavoidable impurity levelconcentration of carbon or nitrogen generally found in the silicon,germanium or silicon-germanium diode material. In one aspect of theinvention, the diode is doped with carbon, nitrogen or a combination ofcarbon and nitrogen in a concentration of at least 1×10¹⁷ cm⁻³, such as1×10¹⁷ to 1×10²¹ cm⁻³, including 1×10¹⁸ to 1×10²⁰ cm⁻³, for example1×10¹⁹ cm⁻³. The carbon and/or nitrogen dopant may be incorporated intothe diode by any suitable method, such as ion implantation, plasmadoping, vapor phase diffusion or in-situ doping during diode layerdeposition.

The diode is preferably a polycrystalline semiconductor diode, such as apolysilicon diode. However, single crystal or amorphous semiconductordiodes may also be used. In some one-time-programmable embodiments, apolycrystalline semiconductor diode is paired with a dielectric ruptureantifuse, though in other embodiments the antifuse may be omitted. Inuse, the diode acts as a read/write element of the nonvolatile memorycell by switching from one resistivity state to a different resistivitystate in response to an applied bias.

FIG. 2 illustrates a memory cell formed according to a preferredembodiment of the present invention. A bottom conductor 12 is formed ofa conductive material, for example tungsten, and extends in a firstdirection. Barrier and adhesion layers may be included in bottomconductor 12. Polycrystalline semiconductor diode 2 has a bottom heavilydoped n-type region 4; an intrinsic region 6, which is not intentionallydoped; and a top heavily doped region 8, though the orientation of thisdiode may be reversed. Such a diode, regardless of its orientation, willbe referred to as a p-i-n diode. Preferably, but not necessarily, thecarbon or nitrogen is incorporated into the intrinsic region 6 such thatin the p-i-n diode 2, at least the intrinsic region 6 of the diode andoptionally the p+ and the n+ regions 4, 8 are doped with the at leastone of carbon or nitrogen. Dielectric rupture antifuse 14 is included insome embodiments. Top conductor 16 may be formed in the same manner andof the same materials as bottom conductor 12, and extends in a seconddirection different from the first direction. Polycrystallinesemiconductor diode 2 is vertically disposed between bottom conductor 12and top conductor 16. Polycrystalline semiconductor diode 2 is formed ina high-resistivity state. This memory cell can be formed above asuitable substrate, for example above a monocrystalline silicon wafer.FIG. 3 shows a portion of a memory level of such devices formed in across-point array, where diodes 2 are disposed between bottom conductors12 and top conductors 16 (antifuses 14 are omitted in this view.)Multiple memory levels can be stacked over a substrate to form a highlydense monolithic three dimensional memory array.

In this discussion a region of semiconductor material which is notintentionally doped is described as an intrinsic region. It will beunderstood by those skilled in the art, however, that an intrinsicregion may in fact include a low concentration of p-type or n-typedopants. Dopants may diffuse into the intrinsic region from adjacentregions, or may be present in the deposition chamber during depositiondue to contamination from an earlier deposition. It will further beunderstood that deposited intrinsic semiconductor material (such assilicon) may include defects which cause it to behave as if slightlyn-doped. Use of the term “intrinsic” to describe silicon, germanium, asilicon-germanium alloy, or some other semiconductor material is notmeant to imply that this region contains no dopants whatsoever, nor thatsuch a region is perfectly electrically neutral.

The resistivity of doped polycrystalline or microcrystallinesemiconductor material, for example silicon, can be changed betweenstable states by applying appropriate electrical pulses. It has beenfound that in preferred embodiments, set transitions are advantageouslyperformed with the diode under forward bias, while reset transitions aremost readily achieved and controlled with the diode under reverse bias.In some instances, however, set transitions may be achieved with thediode under reverse bias, while reset transitions are achieved with thediode under forward bias.

Semiconductor switching behavior is complex. For a diode, both set andreset transitions have been achieved with the diode under forward bias.Generally a reset pulse applied with the diode under forward bias whichis sufficient to switch the polycrystalline semiconductor materialmaking up a diode from a given resistivity state to a higher resistivitystate will be lower amplitude than a corresponding set pulse (which willswitch the same polysilicon semiconductor material from the sameresistivity state to a lower resistivity state) and will have a longerpulse width.

Switching under reverse bias shows a distinct behavior. Suppose apolysilicon p-i-n diode like the one shown in FIG. 2 is subjected to arelatively large switching pulse under reverse bias. After applicationof the switching pulse a smaller read pulse, for example 2 volts, isapplied, and the current flowing through the diode at the read voltage,called the read current, is measured. As the voltage of the switchingpulse under reverse bias is increased in subsequent pulses, thesubsequent read current at two volts changes as shown in FIG. 4. It willbe seen that initially as the reverse voltage and current of theswitching pulse are increased, the read current, when a read voltage isapplied after each switching pulse, increases; i.e. the initialtransition of the semiconductor material (silicon, in this case) is inthe set direction toward lower resistivity. Once the switching pulsereaches a certain reverse bias voltage, at point K in FIG. 4, about−14.6 volts in this example, the read current abruptly begins to drop asreset is achieved and resistivity of the silicon increases. Theswitching voltage at which the set trend is reversed and the silicon ofthe diode begins to reset varies, depending on, for example, theresistivity state of the silicon making up the diode when application ofthe reverse bias switching pulse is begun. It will be seen, then, thatby selecting appropriate voltages, either set or reset of thesemiconductor material making up the diode can be achieved with thediode under reverse bias.

Distinct data states of the memory cell of the embodiments of thepresent invention correspond to resistivity states of polycrystalline ormicrocrystalline semiconductor material making up the diode, which aredistinguished by detecting current flow through the memory cell (betweentop conductor 16 and bottom conductor 12) when a read voltage isapplied. Preferably the current flowing between any one distinct datastate and any different distinct data state is at least a factor of two,to allow the difference between the states to be readily detectable.

The memory cell can be used as a one-time programmable cell or arewriteable memory cell, and may have two, three, four, or more distinctdata states. The cell can be converted from any of its data states toany other of its data states in any order, and under either forward orreverse bias.

Several examples of preferred embodiments will be provided. It will beunderstood, however, that these examples are not intended to belimiting. It will be apparent to those skilled in the art that othermethods of programming a two-terminal device comprising a diode andpolycrystalline or microcrystalline semiconductor material will fallwithin the scope of the invention.

One-Time Programmable Multilevel Cell

In a preferred embodiment of the present invention, a diode formed ofpolycrystalline semiconductor material and a dielectric rupture antifuseare arranged in series disposed between a top and bottom conductor. Thetwo-terminal device is used as a one-time-programmable multilevel cell,in preferred embodiments having three or four distinct data states.

A preferred memory cell is shown in FIG. 2. Diode 2 is preferably formedof a polycrystalline or microcrystalline semiconductor material, forexample silicon, germanium, or an alloy of silicon and/or germanium.Diode 2 is most preferably polysilicon. In this example, bottom heavilydoped region 4 is n-type and top heavily doped region 8 is p-type,though the polarity of the diode may be reversed. The memory cellcomprises a portion of the top conductor, a portion of the bottomconductor, and a diode, the diode disposed between the conductors.

As formed, the polysilicon of diode 2 is in a high-resistivity state,and dielectric rupture antifuse 14 is intact. FIG. 5 a is a probabilityplot showing current of a memory cells in various states. It should benoted that the probability plots shown in FIGS. 5 a, 6, 7, 9 and 11illustrate exemplary embodiments of various programming methods using acell containing a diode that is not doped with carbon or nitrogen. Theprogramming methods for a cell containing a diode that is doped withcarbon or nitrogen are similar to those methods. Turning to FIG. 5 a,when a read voltage, for example 2 volts, is applied between topconductor 16 and bottom conductor 12 (with diode 2 under forward bias)the read current flowing between top conductor 16 and bottom conductor12 is preferably in the range of nanoamps, for example less than about 5nanoamps. Area V on the graph of FIG. 5 a corresponds to a first datastate of the memory cell. For some memory cells in the array, this cellwill not be subjected to set or reset pulses, and this state will beread as a data state of the memory cell. This first data state will bereferred to as the V state.

A first electrical pulse, preferably with diode 2 under forward bias, isapplied between top conductor 16 and bottom conductor 12. This pulse is,for example, between about 8 volts and about 12 volts, for example about10 volts. The current is, for example, between about 80 and about 200microamps. The pulse width is preferably between about 100 and about 500nsec. This first electrical pulse ruptures dielectric rupture antifuse14 and switches the semiconductor material of diode 2 from a firstresistivity state to a second resistivity state, the second state lowerresistivity than the first. This second data state will be referred toas the P state, and this transition is labeled “V→P” in FIG. 5 a. Thecurrent flowing between top conductor 16 and bottom conductor 12 at aread voltage of 2 volts is about 10 microamps or more. The resistivityof the semiconductor material making up diode 2 is reduced by a factorof about 1000 to about 2000. In other embodiments the change inresistivity will be less, but between any data state and any other datastate will be at least a factor of two, preferably at least a factor ofthree or five, and more typically a factor of 100 or more. Some memorycells in the array will be read at this data state, and will not besubjected to additional set or reset pulses. This second data state willbe referred to as the P state.

For example, the read current at 2V can increase from 1×10⁻⁸ A in theunprogrammed state to at least 1×10⁻⁵ A after the programming pulse. Thetable below shows that increasing the programming voltage results in ahigher read current. The last column in the table shows the standarddeviation of the read current.

Programming Programmed Pulse Voltage Read Current @ +2 V 1σ +6.4 V 1.1 ×10⁻⁵ A 6.1 × 10⁻⁶ A +7.4 V 1.7 × 10⁻⁵ A 7.2 × 10⁻⁶ A +8.4 V 1.8 × 10⁻⁵ A5.4 × 10⁻⁶ A

It should be noted that the read currents shown in the table above arefor a cell shown FIG. 2 with the interconnects and a silicon oxideantifuse. If the interconnects are excluded, then the read current iseven higher. For example, for a programming voltage of 8.4V, the readcurrent of the cell without the interconnects is at least 3.5×10⁻⁵ A ata read voltage of at least +1.5V, such as +1.5 to +2V. It is expectedthat further increases in programming voltage would provide a furtherincrease in the read current. For example, increasing the programmingvoltage from 8.4V to 10V is expected to generate an about 70% increasein read current, such that the read current for a cell without theinterconnects is about 6×10⁻⁵ A at 2V read voltage. As noted above,multiple programming pulses, such as 2 to 10 pulses, for example 3-5pulses, may be applied to the diode.

A second electrical pulse, preferably with diode 2 under reverse bias,is applied between top conductor 16 and bottom conductor 12. This pulseis, for example, between about −8 volts and about −14 volts, preferablyabout between about −10 and about −12 volts, preferably about −11 volts.The current is, for example, between about 80 and about 200 microamps.The pulse width is, for example, between about 100 nanosec and about 10microseconds; preferably between about 100 nsec and about 1 microsecond,most preferably between about 200 and about 800 nsec. This secondelectrical pulse switches the semiconductor material of diode 2 from thesecond resistivity state to a third resistivity state, the thirdresistivity state higher resistivity than the second. The currentflowing between top conductor 16 and bottom conductor 12 at a readvoltage of 2 volts is between about 10 and about 500 nanoamps,preferably between about 100 and about 500 nanoamps. Some memory cellsin the array will be read at this data state, and will not be subjectedto additional set or reset pulses. This third data state will bereferred to as the R state, and this transition is labeled “P→R” in FIG.5 a.

FIG. 5 b is a plot of read current versus read voltage for various diodestates illustrated in FIG. 5 a. The diode initially starts in a low readcurrent state V (referred to as the unprogrammed or “virgin” state). Thediode is put the in the programmed state P by the high forward biaspulse, preferably at the factory where the diode is made before theproduct is sold, where power is not a consideration. Once the product issold, the diode is subsequently put in the reset state R by a reversebias programming pulse. The difference between the read currents of theprogrammed and reset states P and R constitutes the “window” for thememory cell, as shown in FIG. 5 b. The large programming voltage and/ormultiple programming pulses allow this window to be as large as possiblefor manufacturing robustness.

To achieve the fourth data state, a third electrical pulse, preferablywith diode 2 under forward bias, is applied between top conductor 16 andbottom conductor 12. This pulse is, for example, between about 8 voltsand about 12 volts, for example about 10 volts, with current betweenabout 5 and about 20 microamps. This third electrical pulse switches thesemiconductor material of diode 2 from the third resistivity state to afourth resistivity state, the fourth resistivity state lower resistivitythan the third, and preferably higher resistivity than the secondresistivity state. The current flowing between top conductor 16 andbottom conductor 12 at a read voltage of 2 volts is between about 1.5and about 4.5 microamps. Some memory cells in the array will be read atthis data state, which will be referred to as the set state S, and thistransition is labeled “R→S” in FIG. 5 a.

The difference in current at the read voltage (for example 2 volts) ispreferably at least a factor of two between any two adjacent datastates. For example, the read current of any cell in data state R ispreferably at least two times that of any cell in data state V, the readcurrent of any cell in data state S is preferably at least two timesthat of any cell in data state R, and the read current of a cell in datastate P is preferably at least two times that of any cell in data stateS. For example, the read current at data state R may be two times theread current at data state V, the read current at data state S may betwo times the read current at data state R, and the read current at datastate P may be two times the read current at data state S. If the rangesare defined to be smaller, the difference could be considerably larger;for example, if the highest-current V state cell can have a read currentof 5 nanoamps and the lowest-current R state call can have a readcurrent of 100 nanoamps, the difference in current is at least a factorof 20. By selecting other limits, it can be assured that the differencein read current between adjacent memory states will be at least a factorof three.

As will be described later, an iterative read-verify-write process maybe applied to assure that a memory cell is in one of the defined datastates after a set or reset pulse, and not between them.

So far the difference between the highest current in one data state andthe lowest current in the next highest adjacent data state has beendiscussed. The difference in read current in most cells in adjacent datastates will be larger still; for example a memory cell in the V statemay have a read current of I nanoamp, a cell in the R state may have aread current of 100 nanoamps, a cell in the S state may have a readcurrent of 2 microamps (2000 nanoamps), and a cell in the P state mayhave a read current of 20 microamps. These currents in each adjacentstate differ by a factor of ten or more.

A memory cell having four distinct data states has been described. Toaid in distinguishing between the data states, it may be preferred forthree rather than four data states to be selected. Four example, athree-state memory cell can be formed in data state V, set to data stateP, then reset to data state R. This cell will have no fourth data stateS. In this case the difference between adjacent data states, for examplebetween the R and P data states, can be significantly larger.

A one-time programmable memory array of memory cells as described, eachcell programmed to one of three distinct data states (in one embodiment)or one of four distinct data states (in an alternative embodiment), canbe programmed as described. These are only examples; clearly there couldbe more than three or four distinct resistivity states and correspondingdata states.

In a memory array of one-time programmable memory cells, the cells maybe programmed in a variety of ways, however. For example, turning toFIG. 6, the memory cell of FIG. 2 may be formed in a first state, the Vstate. A first electrical pulse, preferably under forward bias, rupturesantifuse 14 and switches the polysilicon of the diode from a firstresistivity state to a second resistivity state lower than the first,placing the memory cell in the P state, which in this example is thelowest resistivity state. A second electrical pulse, preferably underreverse bias, switches the polysilicon of the diode from the secondresistivity state to a third resistivity state, the third resistivitystate higher resistivity than the second, placing the memory cell in theS state. A third electrical pulse, preferably also under reverse bias,switches the polysilicon of the diode from the third resistivity stateto a fourth resistivity state, the third resistivity state higherresistivity than the second, placing the memory cell in the R state. Forany given memory cell, any of the data states, the V state, the R state,the S state, and the P state, can be read as a data state of the memorycell. Each transition is labeled in FIG. 6. Four distinct states areshown; there could be three or more than four states as desired.

In still other embodiments, each successive electrical pulse can switchthe semiconductor material of the diode to a successively lowerresistivity state. As in FIG. 7, for example, the memory cell canproceed from the initial V state to the R state, from the R state to theS state, and from the S state to the P state, where for each state theread current is at least two times the read current at the previousstate, each corresponding to a distinct data state. This scheme may bemost advantageous when there is no antifuse included in the cell. Inthis example the pulses may be applied under either forward or reversebias. In alternative embodiments there may be three data states or morethan four data states.

In one embodiment, a memory cell includes the polysilicon ormicrocrystalline diode 2 shown in FIG. 8, including bottom heavily dopedp-type region 4, middle intrinsic or lightly doped region 6, and topheavily doped n-type region 8. As in prior embodiments, this diode 2 canbe arranged in series with a dielectric rupture antifuse, the twodisposed between top and bottom conductors. Bottom heavily doped p-typeregion 4 may be in situ doped, i.e. doped by flowing a gas that providesa p-type dopant such as boron during deposition of the polysilicon, suchthat dopant atoms are incorporated into the film as it forms.

Turning to FIG. 9, it has been found that this memory cell is formed inthe V state, where the current between top conductor 16 and bottomconductor 12 is less than about 80 nanoamps at a read voltage of 2volts. A first electrical pulse, preferably applied under forward biasof, for example, about 8 volts, ruptures dielectric rupture antifuse 14,if it is present, and switches the polysilicon of diode 2 from a firstresistivity state to a second resistivity state, the second resistivitystate lower than the first, placing the memory cell in data state P. Indata state P, the current between top conductor 16 and bottom conductor12 at the read voltage is between about 1 microamp and about 4microamps. A second electrical pulse, preferably applied in reversebias, switches the polysilicon of diode 2 from the second resistivitystate to a third resistivity state, the third resistivity state lowerthan the first. The third resistivity state corresponds to data state M.In data state M, the current between top conductor 16 and bottomconductor 12 at the read voltage is above about 10 microamps. As inprior embodiments, the difference in current between any cell inadjacent data states (the highest-current cell of state V and thelowest-current cell of state P, or between the highest-current cell ofstate P and the lowest-current cell of state M) is preferably at least afactor of two, preferably a factor of three or more. Any of the datastates V, P, or M can be detected as a data state of the memory cell.

FIG. 4 showed that when a semiconductor diode is subjected to reversebias, in general the semiconductor material initially undergoes a settransition to lower resistivity, then, as voltage is increased,undergoes a reset transition to higher resistivity. For this particulardiode, with top heavily doped n-type region 8, and preferably withbottom heavily doped region 4 formed by in situ doping with a p-typedopant, the switch from set transition to reset transition withincreasing reverse bias voltage does not occur as abruptly or as steeplyas with other embodiments of the diode. This means a set transitionunder reverse bias is easier to control with such a diode.

Rewritable Memory Cell

In another set of embodiments, the memory cell behaves as a rewriteablememory cell, which is repeatably switchable between two or between threedata states.

FIG. 10 shows a memory cell that may serve as a rewriteable memory cell.This memory cell is the same as the one shown in FIG. 2, except nodielectric rupture antifuse is included. Most rewriteable embodiments donot include an antifuse in the memory cell, though one may be includedif desired. It should be noted that TiN layers may be included in thecell above and below the diode, as will be described in more detail withrespect to FIGS. 15 a-15 c below.

Turning to FIG. 11, in a first preferred embodiment, the memory cell isformed in a high resistivity state V, with current at 2 volts about 5nanoamps or less. For most rewriteable embodiments the initial V statedoes not serve as a data state of the memory cell. A first electricalpulse, preferably with diode 2 under forward bias, is applied betweentop conductor 16 and bottom conductor 12. This pulse is, for example,between about 8 and about 12 volts, preferably about 10 volts. Thisfirst electrical pulse switches the semiconductor material of diode 2from a first resistivity state to a second resistivity state P, thesecond state lower resistivity than the first. In preferred embodiments,the P state also will not serve as a data state of the memory cell. Inother embodiments, the P state will serve as a data state of the memorycell.

A second electrical pulse, preferably with diode 2 under reverse bias,is applied between top conductor 16 and bottom conductor 12. This pulseis, for example, between about −8 and about −14 volts, preferablybetween about −9 and about −13 volts, more preferably about −10 or −11volts. The voltage required will vary with the thickness of theintrinsic region. This second electrical pulse switches thesemiconductor material of diode 2 from the second resistivity state to athird resistivity state R, the third state higher resistivity than thesecond. In preferred embodiments the R state corresponds to a data stateof the memory cell.

A third electrical pulse can be applied between top conductor 16 andbottom conductor 12, preferably under forward bias. This pulse is, forexample, between about 5.5 and about 9 volts, preferably about 6.5volts, with current between about 10 and about 200 microamps, preferablybetween about 50 and about 100 microamps. This third electrical pulseswitches the semiconductor material of diode 2 from the thirdresistivity state R to a fourth resistivity state S, the fourth statelower resistivity than the third. In preferred embodiments the S statecorresponds to a data state of the memory cell.

In this rewriteable, two-state embodiment, the R state and the S stateare sensed, or read, as data states. The memory cell can repeatedly beswitched between these two states. For example, a fourth electricalpulse, preferably with diode 2 under reverse bias, switches thesemiconductor material of the diode from the fourth resistivity state Sto the fifth resistivity state R, which is substantially the same as thethird resistivity state R. A fifth electrical pulse, preferably withdiode 2 under forward bias, switches the semiconductor material of thediode from the fifth resistivity state R to the sixth resistivity stateS, which is substantially the same as the fourth resistivity state S,and so on. It may be more difficult to return the memory cell to theinitial V state and the second P state; thus these states may not beused as data states in a rewriteable memory cell. It may be preferredfor both the first electrical pulse, which switches the cell from theinitial V state to the P state, and the second electrical pulse, whichswitches the cell from the P state to the R state, to be performedbefore the memory array reaches the end user, for example in a factoryor test facility, or by a distributor before sale. In other embodiments,it may be preferred for only the first electric pulse, which switchesthe cell from the initial V state to the P state, to be performed beforethe memory array reaches the end user.

As will be seen from FIG. 11, in the example provided, the differencebetween current flow under read voltage, for example of 2 volts, betweentop conductor 16 and bottom conductor 12 between any cell in one datastate and any cell in an adjacent data states, in this case the R datastate (between about 10 and about 500 nanoamps) and the S data state(between about 1.5 and about 4.5 microamps), is at least a factor ofthree. Depending on the ranges selected for each data state, thedifference may be a factor of two, three, five, or more.

In alternative embodiments, a rewriteable memory cell can be switchedbetween three or more data states, in any order. Either set or resettransitions can be performed with the diode under either forward orreverse bias.

In both the one-time programmable and rewriteable embodiments described,note that the data state corresponds to the resistivity state ofpolycrystalline or microcrystalline semiconductor material making up adiode. The data states does not correspond to the resistivity state of aresistivity-switching metal oxide or nitride, as in Herner et al., U.S.patent application Ser. No. 11/395,995, “Nonvolatile Memory CellComprising a Diode and a Resistance-Switching Material,” filed Mar. 31,2006, owned by the assignee of the present invention and herebyincorporated by reference.

Reverse Bias Set and Reset

In an array of memory cells formed and programmed according to theembodiments described so far, any step in which cells are subjected tolarge voltages in reverse bias has reduced leakage current as comparedto a forward bias step.

Turning to FIG. 12, suppose 10 volts is to be applied in forward biasacross the selected cell S. (The actual voltage to be used will dependon many factors, including the construction of the cell, dopant levels,height of the intrinsic region, etc.; 10 volts is merely an example.)Bitline B0 is set at 10 volts and wordline W0 is set at ground. Toassure that half-selected cells F (which share bitline B0 with selectedcell S) remain below the turn-on voltage of the diode, wordline W1 isset less than but relatively close to the voltage of bitline B0; forexample wordline W1 may be set to 9.3 volts, so that 0.7 volts isapplied across the F cells (only one F cell is shown, but there may behundreds, thousands or more.) Similarly, to assure that half-selectedcells H (which share wordline W0 with selected cell S) remain below theturn-on voltage of the diode, bitline B1 is set higher than butrelatively close to the voltage of wordline W0; for example bitline B1may be set to 0.7 volts, so that 0.7 volts is applied across cell H(again, there may be thousands of H cells.) The unselected cells U,which share neither wordline W0 or bitline B0 with selected cell S, aresubjected to −8.6 volts. As there may be millions of unselected cells U,this results in significant leakage current within the array.

FIG. 13 shows an advantageous biasing scheme to apply a large reversebias across a memory cell, for example as a reset pulse. Bitline B0 isset at −5 volts and wordline W0 at 5 volts, so that −10 volts is appliedacross selected cell S; the diode is in reverse bias. Setting wordlineW1 and bitline B1 at ground subjects both half-selected cells F and H to−5 volts, at a reverse bias low enough not to cause unintentional set orreset of these cells. Set or reset in reverse bias generally seems totake place at or near the voltage at which the diode goes into reversebreakdown, which is generally higher than −5 volts.

With this scheme, there is no voltage across the unselected cells U,resulting in no reverse leakage. As a result, bandwidth can be increasedsignificantly.

The biasing scheme of FIG. 13 is just one example; clearly many otherschemes can be used. For example bitline B0 can be set at 0 volts,wordline W0 at −10 volts, and bitline B1 and wordline W1 at −5 volts.The voltage across selected cell S, half-selected cells H and F, andunselected cells U will be the same as in the scheme of FIG. 13. Inanother example, bitline B0 is set at ground, wordline W0 at 10 volts,and bitline B1 and wordline W1 each at 5 volts.

Iterative Set and Reset

So far this discussion has described applying an appropriate electricalpulse to switch the semiconductor material of a diode from oneresistivity state to a different resistivity state, thus switching thememory cell between two distinct data states. In practice, these set andreset steps may be iterative processes.

As described, the difference between current flow during read inadjacent data states is preferably at least a factor of two; in manyembodiments, it may be preferred to establish current ranges for eachdata state which are separated by a factor of three, five, ten, or more.

Turning to FIG. 14, as described, data state V may be defined as readcurrent of 5 nanoamps or less at a read voltage of 2 volts, data state Ras read current between about 10 and about 500 nanoamps, data state S asread current between about 1.5 and about 4.5 microamps, and data state Pas read current above about 10 microamps. Those skilled in the art willappreciate that these are examples only. In another embodiment, forexample, data state V may be defined in a smaller range, with readcurrent about 5 nanoamps or less at a read voltage of 2 volts. Actualread currents will vary with characteristics of the cell, constructionof the array, read voltage selected, and many other factors.

Suppose a one-time programmable memory cell is in data state P. Anelectrical pulse in reverse bias is applied to the memory cell to switchthe cell into data state S. In some instances, however, it may be thatafter application of the electrical pulse, the read current is not inthe desired range; i.e. the resistivity state of the semiconductormaterial of the diode is higher or lower than intended. For example,suppose after application of the electrical pulse, the read current ofthe memory cell is at the point on the graph shown at Q, in between theS state and P state current ranges.

After an electrical pulse is applied to switch the memory cell to adesired data state, the memory cell may be read to determine if thedesired data state was reached. If the desired data state was notreached, an additional pulse is applied. For example, when the current Qis sensed, an additional reset pulse is applied to increase theresistivity of the semiconductor material, decreasing the read currentinto the range corresponding to the S data state. As described earlier,this set pulse may be applied in either forward or reverse bias. Theadditional pulse or pulses may have a higher amplitude (voltage orcurrent) or longer or shorter pulse width than the original pulse. Afterthe additional set pulse, the cell is read again, then set or resetpulses applied as appropriate until the read current is in the desiredrange.

In a two-terminal device, such as the memory cell including a diodedescribed, it will be particularly advantageous to read in order toverify the set or reset and to adjust if necessary. Applying a largereverse bias across the diode may damage the diode; thus when performinga set or reset with the diode under reverse bias, it is advantageous tominimize the reverse bias voltage.

Fabrication Considerations

Herner et al., U.S. patent application Ser. No. 11/148,530, “NonvolatileMemory Cell Operating by Increasing Order in PolycrystallineSemiconductor Material,” filed Jun. 8, 2006; and Herner, U.S. patentapplication Ser. No. 10/954,510, “Memory Cell Comprising a SemiconductorJunction Diode Crystallized Adjacent to a Silicide,” filed Sep. 29,2004, both owned by the assignee of the present invention and bothhereby incorporated by reference, describe that crystallization ofpolysilicon adjacent to an appropriate silicide affects the propertiesof the polysilicon. Certain metal suicides, such as cobalt silicide andtitanium silicide, have a lattice structure very close to that ofsilicon. When amorphous or microcrystalline silicon is crystallized incontact with one of these silicides, the crystal lattice of the silicideprovides a template to the silicon during crystallization. The resultingpolysilicon will be highly ordered, and relatively low in defects. Thishigh-quality polysilicon, when doped with a conductivity-enhancingdopant, is relatively highly conductive as formed.

When, in contrast, an amorphous or microcrystalline silicon material iscrystallized not in contact with a silicon having a silicide with whichit has a good lattice match, for example in contact only with materialssuch as silicon dioxide and titanium nitride, with which it has asignificant lattice mismatch, the resulting polysilicon will have manymore defects, and doped polysilicon crystallized this way will be muchless conductive as formed.

In aspects of the present invention, the semiconductor material forminga diode is switched between two or more resistivity states, changing thecurrent flowing through the diode at a given read voltage, the differentcurrents (and resistivity states) corresponding to distinct data states.It has been found that diodes formed of high-defect silicon (or otherappropriate semiconductor materials such as germanium orsilicon-germanium alloys) which has not been crystallized adjacent to asilicide or analogous material providing a crystallization templateexhibit the most advantageous switching behavior.

Without wishing to be bound by any particular theory, it is believedthat one possible mechanism behind the observed changes in resistivityis that set pulses above the threshold amplitude cause dopant atoms tomove out of grain boundaries, where they are inactive, into the body ofa crystal where they will increase conductivity and lower the resistanceof the semiconductor material. In contrast, reset pulses may causedopant atoms to move back to the grain boundaries, lowering conductivityand increasing resistance. It may be, however, that other mechanisms,such as an increase and decrease in degree of order of thepolycrystalline material, are operating as well or instead.

It has been found that the resistivity state of very low-defect siliconcrystallized adjacent to an appropriate silicide cannot be switched asreadily as when the semiconductor material has a higher concentration ofintragrain (e.g. microtwin) defects. It may be that the presence ofdefects, or of a larger number of grain boundaries, allows for easierswitching. In preferred embodiments, then, the polycrystalline ormicrocrystalline material forming the diode is not crystallized adjacentto a material with which it has a small lattice mismatch. A smalllattice mismatch is, for example, a lattice mismatch of about threepercent or less.

Evidence has suggested that switching behavior may be centered onchanges in the intrinsic region. Switching behavior has been observed inresistors and p-n diodes as well, and is not limited to p-i-n diodes,but it is believed that the use of p-i-n diodes may be particularlyadvantageous. The embodiments described so far included a p-i-n diode.In other embodiments, however, the diode may be a p-n diode instead,with little or no intrinsic region.

A detailed example will be provided describing fabrication of apreferred embodiment of the present invention. Fabrication details fromHerner et al., U.S. patent application Ser. No. 10/320,470, “An ImprovedMethod for Making High Density Nonvolatile Memory,” filed Dec. 19, 2002,and since abandoned, hereby incorporated by reference, will be useful information of the diode of these embodiments, as well information fromthe '549 application. Useful information may also be derived from Herneret al., U.S. patent application Ser. No. 11/015,824, “Nonvolatile MemoryCell Comprising a Reduced Height Vertical Diode,” filed Dec. 17, 2004,assigned to the assignee of the present invention and herebyincorporated by reference. To avoid obscuring the invention not all ofthe detail from these applications will be included, but it will beunderstood that no information from these applications is intended to beexcluded.

A fabrication method of a single memory level according to oneembodiment of the invention will be described in detail. Additionalmemory levels can be stacked, each monolithically formed above the onebelow it. In this embodiment, a polycrystalline semiconductor diode willserve as the switchable memory element.

Turning to FIG. 15 a, formation of the memory begins with a substrate100. This substrate 100 can be any semiconducting substrate as known inthe art, such as monocrystalline silicon, IV-IV compounds likesilicon-germanium or silicon-germanium-carbon, III-V compounds, II-VIIcompounds, epitaxial layers over such substrates, or any othersemiconducting material. The substrate may include integrated circuitsfabricated therein.

An insulating layer 102 is formed over substrate 100. The insulatinglayer 102 can be silicon oxide, silicon nitride, high-dielectric film,Si—C—O—H film, or any other suitable insulating material.

The first conductors 200 are formed over the substrate and insulator. Anadhesion layer 104 may be included between the insulating layer 102 andthe conducting layer 106 to help conducting layer 106 adhere toinsulating layer 102. If the overlying conducting layer is tungsten,titanium nitride is preferred as adhesion layer 104.

The next layer to be deposited is conducting layer 106. Conducting layer106 can comprise any conducting material known in the art, such astungsten, or other materials, including tantalum, titanium, copper,cobalt, or alloys thereof.

Once all the layers that will form the conductor rails have beendeposited, the layers will be patterned and etched using any suitablemasking and etching process to form substantially parallel,substantially coplanar conductors 200, shown in FIG. 15 a incross-section. In one embodiment, photoresist is deposited, patterned byphotolithography and the layers etched, and then the photoresist removedusing standard process techniques. Conductors 200 could be formed by aDamascene method instead.

Next a dielectric material 108 is deposited over and between conductorrails 200. Dielectric material 108 can be any known electricallyinsulating material, such as silicon oxide, silicon nitride, or siliconoxynitride. In a preferred embodiment, silicon dioxide is used asdielectric material 108.

Finally, excess dielectric material 108 on top of conductor rails 200 isremoved, exposing the tops of conductor rails 200 separated bydielectric material 108, and leaving a substantially planar surface 109.The resulting structure is shown in FIG. 15 a. This removal ofdielectric overfill to form planar surface 109 can be performed by anyprocess known in the art, such as chemical mechanical planarization(CMP) or etchback. An etchback technique that may advantageously be usedis described in Raghuram et al., U.S. application Ser. No. 10/883,417,“Nonselective Unpatterned Etchback to Expose Buried Patterned Features,”filed Jun. 30, 2004 and hereby incorporated by reference. At this stage,a plurality of substantially parallel first conductors have been formedat a first height above substrate 100.

Next, turning to FIG. 15 b, vertical pillars will be formed abovecompleted conductor rails 200. (To save space substrate 100 is not shownin FIG. 15 b; its presence will be assumed.) Preferably a barrier layer110 is deposited as the first layer after planarization of the conductorrails. Any suitable material can be used in the barrier layer, includingtungsten nitride, tantalum nitride, titanium nitride, or combinations ofthese materials. In a preferred embodiment, titanium nitride is used asthe barrier layer. Where the barrier layer is titanium nitride, it canbe deposited in the same manner as the adhesion layer described earlier.

Next semiconductor material that will be patterned into pillars isdeposited. The semiconductor material can be silicon, germanium, asilicon-germanium alloy, or other suitable semiconductors, orsemiconductor alloys. For simplicity, this description will refer to thesemiconductor material as silicon, but it will be understood that theskilled practitioner may select any of these other suitable materialsinstead.

In preferred embodiments, the pillar comprises a semiconductor junctiondiode. The term junction diode is used herein to refer to asemiconductor device with the property of non-ohmic conduction, havingtwo terminal electrodes, and made of semiconducting material which isp-type at one electrode and n-type at the other. Examples include p-ndiodes and n-p diodes, which have p-type semiconductor material andn-type semiconductor material in contact, such as Zener diodes, andp-i-n diodes, in which intrinsic (undoped) semiconductor material isinterposed between p-type semiconductor material and n-typesemiconductor material.

Bottom heavily doped region 112 can be formed by any deposition anddoping method known in the art. The silicon can be deposited and thendoped, but is preferably doped in situ by flowing a donor gas providingn-type dopant atoms, for example phosphorus, during deposition of thesilicon. Heavily doped region 112 is preferably between about 100 andabout 800 angstroms thick.

Intrinsic layer 114 can be formed by any method known in the art. Layer114 can be silicon, germanium, or any alloy of silicon or germanium andhas a thickness between about 1100 and about 3300 angstroms, preferablyabout 2000 angstroms.

Returning to FIG. 15 b, semiconductor layers 114 and 112 just deposited,along with underlying barrier layer 110, will be patterned and etched toform pillars 300. Pillars 300 should have about the same pitch and aboutthe same width as conductors 200 below, such that each pillar 300 isformed on top of a conductor 200. Some misalignment can be tolerated.

The pillars 300 can be formed using any suitable masking and etchingprocess. For example, photoresist can be deposited, patterned usingstandard photolithography techniques, and etched, then the photoresistremoved. Alternatively, a hard mask of some other material, for examplesilicon dioxide, can be formed on top of the semiconductor layer stack,with bottom antireflective coating (BARC) on top, then patterned andetched. Similarly, dielectric antireflective coating (DARC) can be usedas a hard mask.

The photolithography techniques described in Chen, U.S. application Ser.No. 10/728,436, “Photomask Features with Interior Nonprinting WindowUsing Alternating Phase Shifting,” filed Dec. 5, 2003; or Chen, U.S.application Ser. No. 10/815,312, Photomask Features with ChromelessNonprinting Phase Shifting Window,” filed Apr. 1, 2004, both owned bythe assignee of the present invention and hereby incorporated byreference, can advantageously be used to perform any photolithographystep used in formation of a memory array according to the presentinvention.

Dielectric material 108 is deposited over and between the semiconductorpillars 300, filling the gaps between them. Dielectric material 108 canbe any known electrically insulating material, such as silicon oxide,silicon nitride, or silicon oxynitride. In a preferred embodiment,silicon dioxide is used as the insulating material.

Next the dielectric material on top of the pillars 300 is removed,exposing the tops of pillars 300 separated by dielectric material 108,and leaving a substantially planar surface. This removal of dielectricoverfill can be performed by any process known in the art, such as CMPor etchback. After CMP or etchback, ion implantation is performed,forming heavily doped p-type top region 116. The p-type dopant ispreferably boron or BCl₃. This implant step completes formation ofdiodes 111. The resulting structure is shown in FIG. 15 b. In the diodesjust formed, bottom heavily doped regions 112 are n-type while topheavily doped regions 116 are p-type; clearly the polarity could bereversed.

The carbon and/or nitrogen dopant may be incorporated into the diode byany suitable methods, such as ion implantation, plasma doping, vaporphase diffusion or in-situ doping during diode layer deposition duringany suitable time in the diode fabrication process, preferably prior tothe formation of the upper electrode over the diode. For example, carbonand/or nitrogen may be ion implanted into the diode after the dielectricoverfill planarization step. The carbon and/or nitrogen ion implant maybe conducted before or after the p+ ion implant. Alternatively, if thep+ regions 116 are deposited onto rather than implanted into theintrinsic regions 114, then the carbon and/or nitrogen ion implant maybe conducted after deposition of the intrinsic regions 114 but beforethe deposition of the p+ regions 116. Likewise, the diode region(s) 114and/or 116 may be exposed to a nitrogen and/or carbon containing gas orplasma after the regions 114 and/or 116 are deposited to dope theseregions 114 and/or 116 with carbon and/or nitrogen.

If the carbon and/or nitrogen is doped in situ, then a carbon and/ornitrogen containing gas (such as methane or ammonia, for example) isadded to the silicon or germanium containing source gas (such as silaneor germane) that is used to deposit the silicon, germanium orsilicon-germanium diode layers by CVD, such as LPCVD or PECVD.

Turning to FIG. 15 c, next the optional dielectric rupture antifuselayer 1118 is formed on top of each heavily doped region 116. Antifuse118 is preferably a silicon dioxide layer formed by oxidizing theunderlying silicon in a rapid thermal anneal, for example at about 600degrees. Antifuse 118 may be about 10 to about 100, such as about 20 toabout 40 angstroms thick. Alternatively, antifuse 118 can be deposited.Top conductors 400 can be formed in the same manner as bottom conductors200, for example by depositing adhesion layer 120, preferably oftitanium nitride, and conductive layer 122, preferably of tungsten.Conductive layer 122 and adhesion layer 120 are then patterned andetched using any suitable masking and etching technique to formsubstantially parallel, substantially coplanar conductors 400, shown inFIG. 15 c extending left-to-right across the page. In a preferredembodiment, photoresist is deposited, patterned by photolithography andthe layers etched, and then the photoresist removed using standardprocess techniques.

Next a dielectric material (not shown) is deposited over and betweenconductor rails 400. The dielectric material can be any knownelectrically insulating material, such as silicon oxide, siliconnitride, or silicon oxynitride. In a preferred embodiment, silicon oxideis used as this dielectric material.

Formation of a first memory level has been described. Additional memorylevels can be formed above this first memory level to form a monolithicthree dimensional memory array. In some embodiments, conductors can beshared between memory levels; i.e. top conductor 400 would serve as thebottom conductor of the next memory level. In other embodiments, aninterlevel dielectric (not shown) is formed above the first memory levelof FIG. 15 c, its surface planarized, and construction of a secondmemory level begins on this planarized interlevel dielectric, with noshared conductors.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as a wafer, withno intervening substrates. The layers forming one memory level aredeposited or grown directly over the layers of an existing level orlevels. In contrast, stacked memories have been constructed by formingmemory levels on separate substrates and adhering the memory levels atopeach other, as in Leedy, U.S. Pat. No. 5,915,167, “Three dimensionalstructure memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

A monolithic three dimensional memory array formed above a substratecomprises at least a first memory level formed at a first height abovethe substrate and a second memory level formed at a second heightdifferent from the first height. Three, four, eight, or indeed anynumber of memory levels can be formed above the substrate in such amultilevel array.

An alternative method for forming a similar array in which conductorsare formed using Damascene construction is described in Radigan et al.,U.S. patent application Ser. No. 11/444,936, “Conductive Hard Mask toProtect Patterned Features During Trench Etch,” filed May 31, 2006,assigned to the assignee of the present invention and herebyincorporated by reference. The methods of Radigan et al. may be usedinstead to form an array according to the present invention.

ALTERNATIVE EMBODIMENTS

In addition to those already described, many alternative embodiments ofa memory cell having its data state stored in the resistivity state ofpolycrystalline or microcrystalline semiconductor material are possibleand fall within the scope of the invention. A few other possibleembodiments will be mentioned, but this list cannot and is not intendedto be exhaustive.

FIG. 16 shows a switchable memory element 117 formed in series with adiode 111. The switchable memory element 117 is formed of semiconductormaterial which is switched between resistivity states using electricalpulses as described. The diode is preferably crystallized adjacent to asilicide such as cobalt silicide, which provides a crystallizationtemplate, as described earlier, such that the semiconductor material ofthe diode is very low-defect and exhibits little or no switchingbehavior. Switchable memory element 117 is preferably doped, and shouldbe doped to the same conductivity type as top heavily doped region 116.Methods to fabricate this device are described in U.S. application Ser.No. 11/237,167, filed on Sep. 28, 2005 and incorporated by referenceherein in its entirety.

EXAMPLES

FIGS. 17 a-17 c illustrate probability plots of devices according tonon-limiting examples of the present invention. Specifically, thefigures are probability plots of devices containing 24 memory cells perwafer, showing the reverse leakage current at −5.5V bias at about roomtemperature, after the initial set (or program) and reset operations.FIG. 17 a shows a device of a comparative example in which the diodeswere not intentionally doped with carbon or nitrogen (i.e., there wereno implants into the diode other than the p⁺ implant). The median diodereverse leakage after reset is 4.5×10⁻¹⁰ A. FIG. 17 b shows a device ofan example of the present invention in which the diode received a 50 keVN⁺ implant to a dose of 2×10¹⁵/cm² in addition to the p⁺ implant. Themedian diode reverse leakage after reset is 1.3×10⁻¹⁰ A. FIG. 17 c showsa device of another example of the present invention in which the diodereceived a 50 keV C⁺ implant to a dose of 5×10¹⁵/cm² in addition to thep⁺ implant. The median diode reverse leakage after reset is 3.3×10⁻¹⁰ A.As can be seen from these figures, the reverse or leakage current issubstantially lower for the devices implanted with nitrogen or carbonthan with a device which was not intentionally doped with nitrogen orcarbon. For example, the devices in which the diodes were implanted withcarbon or nitrogen had a leakage current of less than 4×10⁻¹⁰ A (such as1.3 to 3.3×10⁻¹⁰ A) at −5.5V in the high resistivity, reset state, whilethe device of the comparative example had a leakage current of greaterthan 4×10⁻¹⁰ A (4.5×10⁻¹⁰ A) at −5.5V in the high resistivity, resetstate.

Without wishing to be bound by a particular theory, the inventorsbelieve that the carbon or nitrogen species may bond to dangling bondsin the silicon diode passivating them. Reducing or eliminating thedangling bonds as a source of electrically active defects in the diodemay reduce the leakage current.

Detailed methods of fabrication have been described herein, but anyother methods that form the same structures can be used while theresults fall within the scope of the invention. The foregoing detaileddescription has described only a few of the many forms that thisinvention can take. For this reason, this detailed description isintended by way of illustration, and not by way of limitation. It isonly the following claims, including all equivalents, which are intendedto define the scope of this invention. All references, patents andpatent applications mentioned herein are incorporated by reference intheir entirety.

1. A method of making a nonvolatile memory device, comprising: forming afirst electrode; forming at least one nonvolatile memory cell comprisinga silicon, germanium or silicon-germanium diode; doping the diode withat least one of nitrogen or carbon; and forming a second electrode overthe at least one nonvolatile memory cell.
 2. The method of claim 1,wherein in use, the diode acts as a read/write element of thenonvolatile memory cell by switching from a first resistivity state to asecond resistivity state different from the first resistivity state inresponse to an applied bias.
 3. The method of claim 2, wherein thenonvolatile memory cell consists essentially of the diode and the firstand the second electrodes electrically contacting the diode.
 4. Themethod of claim 2, wherein: the nonvolatile memory cell consistsessentially of the first and the second electrodes, the diode and anantifuse; and the diode and the antifuse are located in series betweenthe first and the second electrodes.
 5. The method of claim 1, whereinthe diode is doped with carbon in a concentration of at least 1×10¹⁷cm³.
 6. The method of claim 1, wherein the diode is doped with nitrogenin a concentration of at least 1×10¹⁷ cm³.
 7. The method of claim 1,wherein the diode is doped with carbon and nitrogen in a combinedconcentration of at least 1×10¹⁷ cm³.
 8. The method of claim 1, whereinthe diode comprises a p-i-n diode and at least an intrinsic region ofthe diode is doped with the at least one of carbon or nitrogen.
 9. Themethod of claim 8, wherein the diode comprises a polycrystallinesilicon, germanium or silicon-germanium p-i-n pillar diode having asubstantially cylindrical shape.
 10. The method of claim 8, wherein thestep of doping comprises ion implanting the at least one of carbon ornitrogen into the diode.
 11. The method of claim 10, wherein the leastone of nitrogen or carbon is implanted into the intrinsic region of thediode.
 12. The method of claim 8, wherein the step of doping comprisesin-situ doping the diode with the at least one of carbon or nitrogenduring diode layer deposition.
 13. The method of claim 8, wherein thestep of doping comprises exposing the diode to at least one of a carbonor nitrogen containing plasma during or after diode layer deposition.14. The method of claim 1, further comprising applying a forward bias tothe diode to switch the diode from the first resistivity, unprogrammedstate to the second resistivity, programmed state, wherein the secondresistivity state is lower than the first resistivity state.
 15. Themethod of claim 14, further comprising: applying a reverse bias to thediode to switch the diode to a third resistivity, reset state, whereinthe third resistivity state is higher than the second resistivity state;and applying a forward bias to the diode to switch the diode to a fourthresistivity, programmed set state, wherein the fourth resistivity stateis lower than the third resistivity state.
 16. A method of operating anonvolatile memory device, comprising: providing at least one memorycell which comprises a silicon, germanium or silicon-germanium diodedoped with at least one of carbon or nitrogen, wherein the diode hasbeen switched from a first higher resistivity, unprogrammed state to asecond lower resistivity, programmed state; and applying a reverse biasto the diode to switch the diode to a third resistivity, reset state,wherein the third resistivity state is higher than the secondresistivity state.
 17. The method of claim 16, further comprisingapplying a forward bias to the diode to switch the diode to a fourthresistivity, programmed set state, wherein the fourth resistivity stateis lower than the third resistivity state.
 18. The method of claim 16,further comprising sensing a resistivity state of the diode as a datastate of the memory cell.
 19. The method of claim 16, wherein the diodehas a leakage current of less than 4×10⁻¹⁰ A at −5.5V in the highresistivity, reset state.
 20. The method of claim 16, wherein the diodecomprises a polycrystalline silicon, germanium or silicon-germaniump-i-n pillar diode having a substantially cylindrical shape and thecarbon or nitrogen provide at least one of reduced power, increasedbandwidth or improved temperature characteristics to the memory cellduring read and program operations.